The present invention relates to a photonic switch, and in particular to an apparatus and a method suitable for testing the operation of a photonic switch.
Communications networks are increasingly becoming all optical networks, incorporating photonic (optical) switching. Photonic switches are typically fabricated using Micro Electro-Mechanical systems (MEMS) technology. A recently developed photonic switch of this type is described in xe2x80x9cFree-Space Micro Machined Optical Switches for Optical Networkingxe2x80x9d by LY Lin et al, IEEE Journal of Selected Topics in Quantum Electronics, Vol. 5 No. 1, January/February 1999; which is incorporated herein by reference. Such switches may be used to switch wavelength division multiplexed (WDM) signals as a group, or the WDM signals may be demultiplexed outside the switch and switched individually as channels, or as groups of channels as desired. MEMS switches typically use moveable mirrors to re-direct optical paths within the switch in order to complete an optical signal or channel connection across the switch.
FIG. 1 shows a schematic diagram of a typical MEMS photonic switch 100. The switch 100 is bi-directional, but for simplicity is assumed to comprise 4 inputs in the form of optical fibres 112, 114, 116 and 118, and 4 outputs which are also optical fibres 122, 124, 126 and 128. Each input and output has an associated lens 104 which collimates the beam from each input and focuses the respective beam at each output. Such a switch is generically referred to as a 4xc3x974 switch (number of inputsxc3x97number of outputs).
The switch 100 is a cross point switch, having a switching device (a mirror, 106) located at each of the points at which optical signals emitted from the input fibres would cross with optical signals emitted from the output fibres. The switch 100 thus has a four by four array of mirrors 106 mounted on a surface 102.
In this particular switch, each mirror may be moved between two stable positions. FIGS. 2a and 2b illustrate these positions. FIG. 2a shows the mirror in the inactivated position 106a, where the mirror is flat, i.e. substantially parallel to the surface 102. FIG. 2b shows the mirror having been raised to the activated or upright position 106b, substantially perpendicular to the surface 102. This activation may be performed by a variety of means e.g. by micro actuators causing the mirror to be rotated about the hinges 108. The mirrors are typically formed of materials such as polysilicon, the reflectivity of which is increased by providing a reflective coating 107 such as gold. In the activated state, it is typical for the relatively non reflective surface 109 of the mirror to lie adjacent to the surface 102, so that the reflective coating 107 does not contact the surface 102.
FIG. 1 shows a typical operation of the switch 100. By raising the appropriate mirrors, an optical signal from each of the inputs 112, 114, 116 and 118 is directed to a respective output 128, 126, 122 and 124. For instance, an optical signal originating from input fibre 112 is formed into a collimated beam 132 by lens 104. The beam 132 then reflects off the front reflective surface 107 of a raised mirror 106b into a further lens 104 which focuses the beam 132 into the output fibre 128. It will be appreciated that by appropriate control of the array of mirrors 106, any one of the signals originating from the inputs 112, 114, 116 and 118 can be switched into any one of the outputs, 122, 124, 126 and 128.
Various solutions have been proposed to test the mirror status or switch connection, in order to verify that the mirrors 106 are functioning correctly and are not, for example, jammed in either the raised 106b or flat 106a position.
One solution is to inject different optical test signals into each input port (i.e. 112, 114, 116, 118) to the switch 100 via fibre tap couplers (not shown). Such test signals would be distinct from the normal optical signal being switched e.g. of different wave length and/or modulation characteristics. Each output port (i.e. 122, 124, 126, 128) would then be connected to a further tap coupler. In order that the test signals could be extracted, detected and analysed for verification that the desired input to output connections exist. This solution is true connectivity verification. However, due to the number of components required, it would be both bulky and expensive. For instance, in a Nxc3x97N switch (where N is an integer) the required components would include 2N couplers, N sources, N detectors, as well as numerous splices and fibre interfaces; additionally there would be the assembly cost.
An alternative solution is to use electrical parameters (e.g. capacitance, inductance or resistance) to monitor the physical position of the mirrors. However, this would double the number of electrical connections to the switch matrix, and is hence impractical for large arrays of mirrors.
The present invention aims to address such problems.
In a first aspect, the present invention provides a method of testing the operation of a photonic switch, said switch comprising switching means arranged to be movable between at least a first and a second position, and arranged to switch an incident optical signal by redirection of the optical path of said signal, the method comprising the steps of providing a test optical signal arranged to be incident upon said switching means when in said first position, along a path distinct from the switched optical signal path; and measuring the test signal at a predetermined position suitable for determining if said switching means is in said first position from the measurement. By the term distinct, it is understood that at least a portion of the test optical signal path is different from the switched optical signal path.
Preferably, the method further includes the step of the switching means, when in said first position, redirecting the test optical signal path. Alternatively, the switching means could act to either block (prevent the optical signal reaching a detector) or pass the test signal when in said first position.
Preferably, the redirection occurs as a consequence of at least one of reflection and refraction. Hence a reflective surface such as a mirror or a refractive medium such as glass could be utilised to redirect the signal.
Preferably, the switch comprises a plurality of said switching means, the method steps each being performed a predetermined number of times. For instance, a cross point switch having N inputs and M outputs would have Nxc3x97M switching means, and it could be desirable to check the operation of some or all of the switching means.
Preferably, the method steps are sequentially repeated.
Preferably, the method steps are performed prior to the switch being utilised to switch live optical signals, the method further comprising the steps of sequentially switching said switching means between said first and said second position. For instance, the steps could be performed in order to test the operation of each of the switching means in a recently installed or manufactured switch. Equally, the steps could be performed in order to test the switch operation after a storage or transportation period, prior to the switch being deployed/installed in a system.
Preferably, the switch further comprises a plurality of further switching means arranged to switch a test optical signal along a plurality of paths, each path being incident upon a switching means when in said first position, the method further comprising the step of utilising said further switching means to sequentially provide a test optical signal incident upon the switching means in a predetermined sequence.
Preferably, the method steps are performed while said switch is carrying live optical signals.
In a further aspect, the present invention provides a computer programme arranged to perform a method of testing the operation of a photonic switch, said switch comprising switching means arranged to be movable between at least a first and a second position, and arranged to switch an incident optical signal by redirection of the optical path of said signal the method comprising the steps of providing a test optical signal arranged to be incident upon said switching means when in said first position, along a path distinct from the switched optical signal path; and measuring the test signal at a predetermined position suitable for determining if said switching means is in said first position from the measurement.
Preferably, the computer programme is stored on a machine readable medium.
In another aspect, the present invention provides a photonic switch comprising switching means arranged to be moveable between at least a first and a second position, and arranged to switch an incident optical signal by redirection of the optical path of said signal, the switch further comprising means to provide a test optical signal incident upon said switching means when in said first position along a path distinct from the switched optical signal, and output means suitable for providing an output for measuring the test signal at a predetermined position suitable for determining if said switching means is in said first position from said measurement.
Preferably, the output means comprises an output port suitable for connection to an optical power meter.
Preferably, the switching means has a first reflective surface for redirection of said incident optical signal.
Preferably, the switching means further comprises a second reflective surface arranged to redirect an incident test optical signal along an optical path to said output. The second surface may be only partially reflective. If desired, the surface reflectance characteristics could be enhanced by the addition of an appropriate coating.
Preferably, the first and second reflective surfaces are substantially parallel.
However, appropriate switch configuration could be utilised where the reflective surfaces are not parallel.
Preferably, the switch further comprises N inputs for providing input optical signals to be switched, M outputs for output of the resultant switched signals, and an array of Nxc3x97M switching means for switching said optical signals.
Preferably, one of said inputs is arranged to provide a test optical signal, and one of said outputs is arranged to receive said test optical signal.
Preferably, the switch further comprises an additional input for providing said test signal, an additional output for receiving said output test signal, and an additional row and column of switching means for directing said test signal.
Preferably, the switch further comprises control means arranged to perform a method comprising the steps of providing a test optical signal arranged to be incident upon said switching means when in said first position, along a path distinct from the switched optical signal path; and measuring the test signal at a predetermined position suitable for determining if said switching means is in said first position from the measurement.
In a further aspect the present invention provides a telecommunication system comprising a photonic switch comprising switching means arranged to be moveable to at least a first and a second position, and arranged to switch an incident optical signal by redirection of the optical path of said signal, the switch further comprising means to provide a test optical signal incident upon said switching means when in said first position along a path distinct from the switched optical signal, and output means suitable for providing an output for measuring the test signal at a predetermined position suitable for determining if said switching means is in said first position from said measurement.